I think that he is probably on the right track.However, getting a second opinion would be a good idea. Perhaps only one of those two components actually needs to be replaced.If you can replace just the belt tensioner you will save a whole heap of money.

Is this the original serpentine belt? At 6 years it's due for a change. An old belt can cause all sorts of noises. I've had good luck with Good Year Poly-V belts. A second opinion would be a good idea.